A conventional in-wheel motor driving device 101 is disclosed in JP-A 2009-219271 Gazette (Patent Literature 1) for example.
As shown in FIG. 16, the in-wheel motor driving device 101 includes a housing 102 which is attached to a vehicle body; a motor section 103 which is placed therein and generates a driving force; a wheel hub bearing section 104 which is connected to a wheel; and a speed reducer section 105 which reduces rotation of the motor section 103 and transmits the rotation to the wheel hub bearing section 104, all connected in series.
In the in-wheel motor driving device 101 of the above-described construction, a low-torque high-rotation motor is utilized for the motor section 103 in view of reducing the size of the device. On the other hand, the wheel hub bearing section 104 requires a large torque in order to drive the wheel. For these reasons, a cycloid reduction gear system is often utilized for the speed reducer section 105 due to its compactness and high speed-reduction ratio.
A speed reducer section 105 utilizing a cycloid reduction gear system includes a motor-side rotation member 106 which has eccentric sections 106a, 106b; cycloid discs 107a, 107b which are disposed in the eccentric sections 106a, 106b; roller bearings 106c which rotatably support the cycloid discs 107a, 107b with respect to the motor-side rotation member 106; a plurality of outer circumferential engagers 108 which make engagement with outer circumferential surfaces of the cycloid discs 107a, 107b to generate rotational movement of the cycloid discs 107a, 107b; and a plurality of inner pins 109 which transmit the rotational movement of the cycloid discs 107a, 107b to the wheel-side rotation member 110.